ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Accruing and Nonaccruing Past Due Amounts (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Current	$ 126,414	$ 117,946
Total	127,528	119,088
Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	497	511
Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	198	212
Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	419	419
Commercial
Financing Receivable, Past Due [Line Items]
Current	65,834	57,358
Total	66,032	57,538
Commercial | Commercial
Financing Receivable, Past Due [Line Items]
Current	48,909	41,340
Total	49,092	41,479
Commercial | Commercial real estate
Financing Receivable, Past Due [Line Items]
Current	14,494	13,520
Total	14,502	13,522
Commercial | Leases
Financing Receivable, Past Due [Line Items]
Current	2,431	2,498
Total	2,438	2,537
Commercial | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	104	83
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	97	45
Commercial | Financing Receivables, 30 to 59 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	1	1
Commercial | Financing Receivables, 30 to 59 Days Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	6	37
Commercial | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	34	28
Commercial | Financing Receivables 60 To 89 Days Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	28	27
Commercial | Financing Receivables 60 To 89 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	6	1
Commercial | Financing Receivables 60 To 89 Days Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Commercial | Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	60	69
Commercial | Financing Receivables 90 Days or More Past Due | Commercial
Financing Receivable, Past Due [Line Items]
Past Due	58	67
Commercial | Financing Receivables 90 Days or More Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Past Due	1	0
Commercial | Financing Receivables 90 Days or More Past Due | Leases
Financing Receivable, Past Due [Line Items]
Past Due	1	2
Retail
Financing Receivable, Past Due [Line Items]
Current	60,580	60,588
Total	61,496	61,550
Retail | Residential mortgages
Financing Receivable, Past Due [Line Items]
Current	18,586	18,947
Total	18,721	19,083
Retail | Home equity
Financing Receivable, Past Due [Line Items]
Current	12,681	12,834
Total	12,992	13,154
Retail | Automobile
Financing Receivable, Past Due [Line Items]
Current	11,856	11,788
Total	12,157	12,120
Retail | Education
Financing Receivable, Past Due [Line Items]
Current	10,835	10,290
Total	10,887	10,347
Retail | Other retail
Financing Receivable, Past Due [Line Items]
Current	6,622	6,729
Total	6,739	6,846
Retail | Financing Receivables, 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	393	428
Retail | Financing Receivables, 30 to 59 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	25	35
Retail | Financing Receivables, 30 to 59 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	92	91
Retail | Financing Receivables, 30 to 59 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	206	227
Retail | Financing Receivables, 30 to 59 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	24	30
Retail | Financing Receivables, 30 to 59 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	46	45
Retail | Financing Receivables 60 To 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Past Due	164	184
Retail | Financing Receivables 60 To 89 Days Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	16	17
Retail | Financing Receivables 60 To 89 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	34	40
Retail | Financing Receivables 60 To 89 Days Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	72	81
Retail | Financing Receivables 60 To 89 Days Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	13	15
Retail | Financing Receivables 60 To 89 Days Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	29	31
Retail | Financing Receivables 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Past Due	359	350
Retail | Financing Receivables 90 Days or More Past Due | Residential mortgages
Financing Receivable, Past Due [Line Items]
Past Due	94	84
Retail | Financing Receivables 90 Days or More Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Past Due	185	189
Retail | Financing Receivables 90 Days or More Past Due | Automobile
Financing Receivable, Past Due [Line Items]
Past Due	23	24
Retail | Financing Receivables 90 Days or More Past Due | Education
Financing Receivable, Past Due [Line Items]
Past Due	15	12
Retail | Financing Receivables 90 Days or More Past Due | Other retail
Financing Receivable, Past Due [Line Items]
Past Due	$ 42	$ 41